Myopia (near-sightedness) is the most common optical refractive error. Over 133 million individuals in the United States wear some form of refractive correction, either in the form of spectacles or contact lenses.
Recently, surgical correction of refractive errors, or refractive surgery, has gained increasing popularity. The most popular form of refractive surgery is called radial keratotomy (RK). In this surgery, a series of radial incisions of precise depth are placed in the cornea, reflecting a change in its curvature and hence its refractive power. RK surgery was initially regarded with great skepticism by the ophthalmic community, however its effectiveness and relative safety have been demonstrated in the last few years. Accordingly, an increasing number of ophthamologists have been performing this operation.
The length and number of incisions required in an RK operation varies according to the degree of optical correction desired, and the depth of the incision is critical to the satisfactory outcome of the surgery. Inadequate depth will result in undercorrection, whereas excessive depth will result in overcorrection or perforation of the cornea and potentially serious harm to the eye. Accurate measurement of the corneal thickness is therefore essential to safe and successful RK surgery.
Instruments that measure corneal thickness are referred to as pachymeters. Early pachymeters were purely optical devices and had been shown to be imprecise. Ultrasonic technology is now routinely used to measure corneal thickness. All current instruments employ an ultrasonic probe which contacts the eye and which is connected via cable to a desk unit which is about the size of an oscilloscope. Some units have a smaller oscilloscope screen that displays the transducer tracings while other more recent units simply display a digital readout of the corneal thickness.
The most commonly performed operation in ophthalmology is cataract surgery. A cataract is an opacification of the biological lens inside the eye. In cataract extraction, one of the several techniques is used to remove the opacified lens material. Once removed, it is possible to implant an artificial lens in order to restore optical integrity to the eye, eliminating the need for thick cataract glasses or contact lenses.
When cataract surgery is performed, several measurements are necessary in order to calculate the precise power of the intraocular lens to be implanted. The important variables are corneal curvature (keratometry), size of the eye (axial length), knowledge of where inside the eye the inner ocular lens is to be implanted (interior or posterior chamber). Measurement of axial length is performed ultrasonically by a device referred to as a biometric ruler. Through such an instrument, the major internal structures of the eye can be imaged and their dimensions measured. Of importance in the measurement of true axial length is the distance from the cornea to the retina along the visual axis of the eye. The early biometric rulers employed a piezoelectric transducer, in a hand-held probe attached via a cable to an oscilloscope. It was necessary for the individual performing the examination to visually assess the oscilloscope signals and identify an axial scan. Second generation instruments still require the examiner to detect the pattern consistent with an axial scan. Then, electronics are used to calculate and display the desired measurement in millimeters. It would be desirable to have a self-contained, hand-held, digital, ultrasonic biometric ruler instrument for displaying the axial length of the eye using microprocessor technology to assess the echo waveforms and displaying digitally a readout representing as accurately as possible the true axial length of the eye.